How to make the top part of a render less saturated/more pale?
You can do in compositing nodes.

Here is an example:



Node Hue Saturation Value can make your image more/less saturated, but you need to add a mask to specify on that part of image the effect will be affects. You can draw a mask, draw an image or like in my case, use a box mask node and blur it.

If you familiar with masks, you can draw a mask with feather, and use it, it gives more control of areas witch must be saturated: